The present invention relates to improvements in digital to analog (D to A) signal converters and more particularly to an exponential D to A converter particularly useful in a system including an implanted cochlear electrode for stimulating the cochlea of a hearing impaired person.
The purpose of an exponential D to A signal converter is to convert a digital input signal consisting of a series of digital pulses to an analog output signal the amplitude of which increases in steps where each step is a percentage of the prior step. Various circuits haVe been developed to accomplish that purpose. For a system of the above-described type it has been proposed that the percentage be about 31/2 percent. To accomplish this, a converter as illustrated in FIG. 1 has been proposed including a series of stages each comprising a current mirror. In FIG. 1, the current mirror of Stage 1 comprises P-type CMOS input and output transistors Q1 and Q3 while the current mirror of Stage 2 comprises N-type CMOS input and output transistors Q5 and Q7. The gate and drain of the input transistor Q1 are connected to a reference current source while the sources of transistors Q1 and Q3 are connected to a relatively positive supply voltage. The gate and drain of the input transistor Q5 are connected to the output of Stage 1 while the sources of the transistors Q5 and Q7 are connected to a relatively negative supply voltage. A series circuit comprising a current adjusting transistor and a transistor switch is connected in parallel with each input and output transistor. In Stage 1, the P-type CMOS transistors Q2 and Q4 comprise the current adjusting transistors while the P-type CMOS transistors Q2S and Q4S comprise the switching transistors. In Stage 2, the N-type CMOS transistors Q6 and Q8 comprise the current adjusting transistors while the N-type CMOS transistors Q6S and Q8S comprise the switching transistors. The purpose of the series circuits is to slightly increase or decrease the output current of the current mirror to which they are connected. For example, activation of Q2 by a turning on of Q2S decreases slightly the current output of Stage 1, preferably by about 31/2 percent. When Q4 is activated by a turning on of Q4S, the current output of Stage 1 is increased, preferably by about 7 percent. As illustrated, the transistor switches are under the control of digital pulses applied to input terminals connected to the gates of the transistors Q2S, Q4S, Q6S and Q6S. Thus for Stage 1, by application of digital pulses to the input terminals of the transistors Q2S and Q4S, the output current of Stage 1 may be stepped by increments of about -31/2, 0, +31/2 and 7 percent. In a similar manner, the output current of Stage 2 may be stepped by increments of about 15 and 30 percent.
In practice it has been found that the converter of FIG. 1 presents certain undesirable problems. First, the switching transistors possess some resistance and being in series with the sources of the current adjusting transistors, the source to drain currents of the current adjusting transistors flow therethrough. Thus, the switching transistors introduce voltage drops that are reflected as changes in the gate voltages of the adjacent current adjusting transistors and undesired changes in the output currents of the current mirrors. Second, in the circuit of FIG. it is very difficult to predictably control the output currents. This is because the input and output transistors comprising the current mirrors do not operate under the same conditions. In particular, the drain voltages of the input and output transistors are not maintained constant. For example, the drain voltage of the input transistor Q5 is substantially different than the drain voltage of the output transistor Q7. For the transistors to be matched in operation requires that they operate at substantially the same drain voltages.
The D to A converter of the present invention overcomes the foregoing problems of the circuit of FIG. 1 and represents a substantial improvement in D to A converters particularly those useful in implantable systems for stimulating the cochlea of the hearing impaired.